In a mechanical device such as a speed reducer, a housing that houses mechanical parts such as a gear and a bearing contains a lubricant to prevent damage to the mechanical parts. The lubricant includes an abrasion powder (mainly iron powder) mixed therein as the mechanical parts wear during operation of the mechanical device.
In general, when wear of mechanical parts advances into the wear-out failure period in the failure rate curve (the bathtub curve), a larger amount of abrasion powder (produced from the mechanical parts) is mixed into the lubricant. For preventive maintenance, it is necessary to timely sense the increase of the amount of produced abrasion powder.
For example, Japanese Patent Application Publication No. 2005-331324 (“the '324 Publication”) discloses a sensor that senses the amount of metal powder in an oil. The sensor of the '324 Publication includes: a sensor head having a permanent magnet; a cup-shaped electrode provided on a distal end surface of the sensor head; and a plurality of rod-shaped conductive members arranged on an outer peripheral surface of the sensor head. The output of the sensor is varied when a short circuit occurs between the rod-shaped conductive members due to the abrasion powder accumulated between opposed end surfaces of the conductive members and the cup-shaped electrode subjected to a magnetic field by the permanent magnet (a sensing region). In the '324 Publication, the uncleanness of the oil can be sensed by variation of the output of the sensor.
In the '324 Publication, the magnetic flux leaks into spaces around the sensing region to cause the abrasion powder to be accumulated in regions other than the sensing region. Therefore, a small amount of abrasion powder accumulates in the sensing region, resulting in lower sensitivity of the sensor.